The US military plans to send hundreds more spies overseas as part of an ambitious plan that will more than double the size of its espionage network, it was reported Sunday.

The Defense Intelligence Agency (DIA), the Pentagon's military intelligence unit, is aiming to recruit 1,600 intelligence "collectors" – up from the several hundred overseas agents it has employed in recent years, sources told The Washington Post.

Combined with the enormous growth in the CIA since 9/11 attacks, the recruitment drive will create an unprecedented spy network. "The stars have been aligning on this for a while," an anonymous former senior US military official involved in planning the DIA transformation told the Post.

The news is likely to heighten concerns about the accountability of the US military's clandestine programmes amid mounting concerns about the CIA-controlled drone programme.

The United Nations said last month that it intends to investigate civilian deaths from drone strikes. The US has refused to even acknowledge the existence of a drone programme in Pakistan. The US military is not subject to the same congressional notification requirements as the CIA, creating yet more potential controversies.

With the US pulling out of Afghanistan and operations in Iraq winding down, government officials are looking to change the focus of the DIA away from battlefield intelligence and to concentrate on gathering intelligence on issues including Islamist militant groups in Africa, weapons trades in North Korea and Iran, and the military build up in China.

"It's the nature of the world we're in," said the senior defense official, who is involved in overseeing the changes at the DIA. "We just see a long-term era of change before things settle."

The DIA's new recruits would include military attachés and others who do not work undercover. But US officials told the Post that the growth will be driven a new generation of spies who will take their orders from the Department of Defense.

The DIA is increasingly recruiting civilians to fill out its ranks as it looks to place agents as academics and business executives in militarily sensitive positions overseas.

Officials said the sheer number of agents that the DIA is looking to recruit presents its own challenge as the agency may struggle to find enough overseas vacancies for its clandestine agents. "There are some definite challenges from a cover perspective," a senior defense official said.

The news comes as the Obama administration faces growing criticism about its use of CIA drones to target enemies overseas. The drone programme will continue under the aegis of the CIA. The DIA agents will concentrate on military intelligence, tracking aircraft development for example, and will report findings to the CIA.

The recruitment drive comes after a decade of enormous growth period at the CIA following the terrorist attacks of September 11, 2001. Since the 9/11 attacks the CIA's counterterrorism center has grown from 300 to over 2,000 agents.

But despite the hiring bonanza, officials said that the agency has become overstretched as its activities worldwide have broadened. Hundreds of military assignments are expected to be turned over to the newly arrived DIA operatives.

"The CIA doesn't want to be looking for surface-to-air missiles in Libya" when it's also under pressure to assess the opposition in Syria, a former high-ranking US military intelligence officer told The Post.

The plan does face opposition in Washington, where critics believe its terms are overly generous to the CIA. Turf wars broke out between the two intelligence agencies after previous efforts by the Pentagon to expand its intelligence role — particularly during Donald Rumsfeld's time as defense secretary.

This time the project is being driven by former CIA agents including Michael Vickers, the top intelligence official at the Pentagon and a CIA veteran, and Leon Panetta, a former CIA director and the current secretary of the Defense Department.